Blood Clotting Flashcards
Which coagulation test needs to be looked at for a patient on heparin?
Partial Thromboplastin Time (PTT), which measures the intrinsic pathway.
What is thrombocytopenia?
Too few platelets.
Which coagulation test needs to be looked at for a patient on warfarin?
Prothrombin Time (PT) which looks at the extrinsic pathway, and is used in the INR measurement.
Name two clotting factors that circulate in the blood.
Factor VII, and factor X.
What is thrombocytopenia?
Low circulating platelets.
What is Glanzmann’s thrombasthenia?
An autosomal recessive condition which affects megakaryocytes and results in a lack of platelet aggregation. It can be deected using platelet aggregometry.
What is haemophilia A?
Deficiency in factor 8.
What is haemophilia B?
Deficiency in factor 9.
How is the intrinsic pathway of coagulation initiated?
There is damage to the wall of the blood vessel (can be atherosclerosis caused by scavenger macrophages absorbing too much LDL) which causes factor XII in the blood to be activated.
This activates factor XI, which activates factor IX, which forms a complex with factor VIII, which activates factor X, which produces prothrombinase with factor V.
What is haemostasis?
The maintenance of fluid blood flow, by a clot forming to stop the loss of blood but maintain blood flow through the vessel and prevent infarction.
What is thrombosis?
Pathological manifestation of haemostasis, causing hypoxia and tissue damage.
What is the final stage of haemostasis called, when the fibrin fibres pull the platelet plug into a more robust clot and the damaged ends of the vessel wall together?
Clot retraction.
What is thrombocythaemia?
Too high platelet count.
What is TTP and what causes it?
Thrombotic Thrombocytopenic Purpura. It is a rare side effect of clopidogrel (anti-thrombotic drug which inhibits platelet aggregation by blocking the pyrimidine receptor on platelets so the platelet agonist ADP can’t bind and activate them - also happens to inactivate the enzyme that breaks up von Willebrand Factor produced by endothelial cells, which instead forms large molecules that platelets bind to and form microemboli).
Name the three stages of haemostasis.
Vascular spasm, primary haemostasis (platelet plug formation), secondary haemostasis (coagulation).
What is contained in the alpha granules of platelets?
Adhesion proteins e.g von Willebrand Factor, platelet specific proteins, membrane proteing (glycoproteins).
What is contained in the dense (darkly staining) granules of platelets?
Vasoconstrictive agents (e.g serotonin), platelet agonists (ADP, ATP, thromboxane), Ca2+ and Mg2+.
What is the name of the receptors on the platelet membranes that bind to the platelet agonist ADP?
Pyrimidine receptors.
Why do the binding of collagen and von Willebrand factor etc cause an immediate shape change in platelets?
The receptors these adhesion proteins bind to are directly connected to cytoskeletal proteins inside the platelet.
What is the most important factor causing platelets to roll along the subendothelium and expose more of their glycoproteins to it for adhesion?
The sheer mechanical force of the blood flow.
What is another name for clotting factors (inactive precursors of enzymes)?
Zymogens.
What causes the extrinsic pathway of coagulation to be initiated?
Damage to cells outside of the blood vessel which causes them to release Tissue Factor, which activates and forms a complex with factor VII, which activates factor X and V to form prothrombinase.
What happens in the common pathway of coagulation?
Prothrombinase converts prothrombin to thrombin. Thrombin then converts soluble fibrinogen to insoluble fibrin. In the presence of activated factor XIII, fibrin forms a cross-linked fibrin clot, and clot retraction occurs.
Why is there a change in the light absorbed by the platelet rich plasma after it has been activated in platelet aggregometry?
At first the light absorbed increases as the platelets change shape and the mixture is more opaque, then the light absorbed decreases as the platelets form a plug and the light is able to travel through the colourless plasma.
